There is a growing demand for alternative renewable energy technologies due to the shortage of fossil energy resources and environmental concerns. Thus, various devices capable of harvesting energy from sustainable resources such as light, heat, and mechanical vibrations have been investigated. These include piezoelectric and triboelectric nanogenerators (NGs) that are capable of effectively harvesting ubiquitous mechanical energy from the natural environment and converting it to electric energy. For example, ZnO-nanowire-based NGs capable of converting mechanical energy to electrical power have been investigated. (See, for example, Xu et al., Nat Nano 5 (2010) 366-373; and Wang, et al., Science 312 (2006) 242-246.) The development of ZnO-nanowire-based NGs has reached an output power of sub-milliwatt levels, which is sufficient to power many small electronic devices. However, an electric poling step is essential for these traditional piezoelectric materials in order to align the electric dipoles to achieve high piezoelectric outputs. (See, X. Wang, Nano Energy 1 (2012) 13-24.) Furthermore, such NGs normally require sophisticated growth and fabrication procedures, thereby limiting their mechanical flexibility as well as their potential applications.
Triboelectric NGs, another technology for energy harvesting, can have higher output performance and lower cost relative to conventional piezoelectric NGs. The output power density of triboelectric NGs has been reported at the milliwatt per cm2 level. However, existing triboelectric NGs require an airgap between contacting material pairs within the device assembly in order to induce a potential difference. The presence of the airgap not only requires a complicated fabrication and packaging process, but also leads to unfavorable durability and stability, which may limit the large-scale manufacturing of conventional triboelectric NGs for practical applications.
Various perovskite materials, including BaTiO3 (BTO), Pb(Zr,Ti)O3 (PZT), and (K, Na)NbO3 (KNN) have also been utilized to enhance the output power generation of NGs.
NG flexibility is one of the key considerations in mechanical energy harvesting applications, and is often achieved by making a composite of an inorganic piezoelectric material and an elastomeric polymer. (See, for example, Lin et al., The Journal of Physical Chemistry Letters 3 (2012) 3599-3604; Jeong et al., Adv Funct Mater 24 (2014) 2620-2629; Joung et al., Journal of Materials Chemistry A 2 (2014) 18547-18553; and Park et al., Adv Mater 24 (2012) 2999-3004.) However, the inorganic piezoelectric materials in the polymer composites are prone to settling to the bottom of the composite layers, thereby significantly weakening the performance of the NGs.